Japanese Laid-Open Patent Publication No. 56-16779 describes a key for insertion into an associated cylinder lock. When the proper key is inserted in the cylinder lock, the key and the cylinder lock are engaged with each other so as to satisfy a predetermined concavo-convex relationship. This enables the turning of the key to, for example, open the lock. Such a key (hereafter, referred to as the first key) has a blade. One side of the blade has a wall functioning as a guide for guiding the insertion of the blade into the cylinder lock. The other side of the blade includes an engaging portion for engaging tumblers in the cylinder lock. Serrations are formed along the engaging portion. The serrations are formed to engage the tumblers and satisfy the predetermined concavo-convex relationship when the key is inserted into the cylinder lock. The first key is difficult to duplicate due to the wall provided on one side of the blade. However, the first key must always be inserted into the cylinder lock with the wall facing the same direction. In other words, the first key is irreversible.
FIGS. 7A and 7B show another known key (hereafter, referred to as the second key 101) for insertion into an associated cylinder lock. The turning of the second key 101 is enabled when the second key 101 and the cylinder lock are engaged with each other so as to satisfy a predetermined concavo-convex relationship. The second key 101 includes a groove 103 formed in each side of a blade 102. Serrations 104 are formed along the walls defining each groove 103. The groove 103 and serrations 104 on one side of the blade 102 are symmetric to the groove 103 and serrations 104 on the other side of the blade 102. Thus, the second key 101 is reversible.
In the second key 101, a wall 105 extends between the bottom portions of the two grooves 103. The wall 105 makes it difficult to duplicate the second key 101. However, the wall 105 increases the thickness of the blade 102.